Electro-wetting on dielectric (EWOD) is a well known technique for manipulating droplets of fluid by application of an electric field. Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in an active matrix array incorporating transistors, for example by using thin film transistors (TFTs). EWOD (or AM-EWOD) is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 72, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of electrodes 38 (e.g., 38A and 38B in FIG. 1) are realized. The electrode of a given array element may be termed the element electrode 38. The liquid droplet 4, including a polar material (which is commonly also aqueous and/or ionic), is constrained in a plane between the lower substrate 72 and a top substrate 36. A suitable gap between the two substrates may be realized by means of a spacer 32, and a non-polar fluid 34 (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet 4. An insulator layer 20 disposed upon the lower substrate 72 separates the conductive element electrodes 38A, 38B from a first hydrophobic coating 16 upon which the liquid droplet 4 sits with a contact angle 6 represented by θ. The hydrophobic coating is formed from a hydrophobic material (commonly, but not necessarily, a fluoropolymer).
On the top substrate 36 is a second hydrophobic coating 26 with which the liquid droplet 4 may come into contact. Interposed between the top substrate 36 and the second hydrophobic coating 26 is a reference electrode 28.
The contact angle θ 6 is defined as shown in FIG. 1, and is determined by the balancing of the surface tension components between the solid-liquid (γSL), liquid-gas (γLG) and non-ionic fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by:
                              cos          ⁢                                          ⁢          θ                =                                            γ                              S                ⁢                                                                  ⁢                G                                      -                          γ                              S                ⁢                                                                  ⁢                L                                                          γ                          L              ⁢                                                          ⁢              G                                                          (                  equation          ⁢                                          ⁢          1                )            
In operation, voltages termed the EW drive voltages, (e.g. VT, V0 and V00 in FIG. 1) may be externally applied to different electrodes (e.g. reference electrode 28, element electrodes 38A and 38B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic coating 16. By arranging for different EW drive voltages (e.g. V0 and V00) to be applied to different element electrodes (e.g. 38A and 38B), the liquid droplet 4 may be moved in the lateral plane between the two substrates 72 and 36.
FIG. 1B shows a circuit representation of the electrical load presented between the element electrode 38 and the reference electrode 28. The liquid droplet 4 can be modeled as a resistor and capacitor in parallel, the hydrophobic coatings 16 and 26 as capacitors and the insulator 16 as a capacitor. For the purposes of driving and sensing, the electrical load functions effectively as a capacitor whose value depends on whether a liquid droplet 4 is present or not a given element electrode 38.
U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses a passive matrix EWOD device for moving droplets through an array.
U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions.
U.S. Pat. No. 6,565,727 further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials.
U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based thin film electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electro-wetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based thin film electronics to control an EWOD array, namely:                Electronic driver circuits can be integrated onto the lower substrate 72.        TFT-based thin film electronics are well suited to the AM-EWOD application.        
They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost.                TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require EWOD actuation voltages in excess of 20V to be applied.        
A disadvantage of U.S. Pat. No. 7,163,612 is that it does not disclose any circuit embodiments for realizing the TFT backplane of the AM-EWOD.
EP2404675 (Hadwen et al., published Jan. 11, 2012) describes array element circuits for an AM-EWOD device. Various methods are known for programming and applying an EWOD actuation voltage to the EWOD element electrode 38. The voltage write function described includes a memory element of standard means, for example, based on Dynamic RAM (DRAM) or Static RAM (SRAM) and input lines for programming the array element.
US Application US20100096266 (Kim et al., published Apr. 22, 2010) describes an EWOD device having a reservoir site that is configured to hold a quantity of liquid. Droplets are dispensed from the reservoir using control circuitry with a feedback mechanism. The control circuitry is configured to measure the fluid volume on the electrodes and independently adjust an applied voltage to increase/decrease the quantity of fluid.
U.S. Pat. No. 7,439,014 (Pamula et al., issued Oct. 21, 2008) describes a method for effecting serial dilution on an EWOD device by combining a droplet with a droplet of wash buffer and then splitting the resultant droplet into two parts.
US Application US20130115703 (Bhattacharya et al., published May 9, 2013) describes a method for producing fluids with desired concentration factors by sequences of mix/split steps on an EWOD device.
Various methods are known for detecting the position and size of one or more droplets on an EWOD device. US8872527 (Sturmer et al., issued Oct. 28, 2014) describes a method for capacitance detection on an EWOD droplet actuator.
US8653832 (Hadwen et al., issued Feb. 18, 2014) describes how an impedance (capacitance) sensing function can be incorporated into the array element of an AM-EWOD device. The sensor function may be utilized to measure the position of one or more droplets on the array. The sensor function may further be utilized to measure the size of droplets which may overlap one or more elements of the array. A method of determining droplet size from sensor data is described in “Programmable large area digital microfluidic array with integrated droplet sensing for bioassays”, Hadwen et al, Lab Chip. 2012 Sep. 21; 12(18):3305-13.
Digital polymerase chain reaction (dPCR) is a method for measuring the quantity of a target nucleic acid sequence in a sample of interest. The basic method is described in the article “Digital PCR hits its stride”, Nature methods Vol. 9 No. 6 p 541, and involves the sample being diluted and partitioned into hundreds or even millions of separate reaction chambers so that each contains one or no copies of the sequence of interest. By counting the number of ‘positive’ partitions (in which the sequence is detected) versus ‘negative’ partitions (in which it is not), one can determine exactly how many copies of a DNA molecule were in the original sample.